Coated paper for printing

ABSTRACT

A coated printing paper offering high gloss, no mottled impression, sufficiently practical levels of printability in terms of ink-drying property and surface peel strength, and good productivity achieved by elimination of deposits on the calender rolls, wherein the coated printing paper comprises a base paper and a coating layer comprising pigment and adhesive formed on the base paper, and wherein a surface layer comprising 100 weight-parts of thermoplastic copolymer with a glass-transition temperature of 80° C. or above and 3 to 100 weight-parts of surface sizing agent of styrene-acrylic, olefin or styrene-maleic acid copolymer type is formed on the coating layer.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP02/03186, filed on Mar. 29, 2002, whichclaims priority of Japanese Patent Application Nos. 2001-097595 filed onMar. 29, 2001 and 2002-090076 filed on Mar. 28, 2002. The InternationalApplication was published under PCT Article 21(2) in a language otherthan English.

FIELD OF THE INVENTION

This invention relates to a high-gloss coated paper offering excellentprintability.

Specifically, the invention relates to a coated printing paper having asurface layer formed by applying and drying a mixed solution ofthermoplastic copolymer and surface sizing agent, which produces nomottled impression, provides sufficiently practical levels ofprintability in terms of ink-drying property and surface peel strength,and leaves no deposits on the calender rolls.

BACKGROUND OF THE INVENTION

Coated papers, which have a coating layer comprising pigment and binder,are used as high-quality printing papers. As such, in addition toprintability in terms of ink absorbency and strength of coating layer,it is also important for coated papers to achieve sufficient gloss onthe surface of coating layer. However, pressure-smoothing the surface ofcoating layer for the purpose of achieving higher gloss will inevitablycrush the voids in the coating layer and thereby reduce the paper'sink-absorbing capability. In addition, the use as a pigment binder of alarge amount of water-soluble or water-dispersant polymer substance,such as copolymer latex, will improve the strength and gloss of thecoating layer, but it will also decrease the voids in the coating layerand thereby reduce the paper's ink-absorbing capability. In this sense,gloss and printability are mutually exclusive. Accordingly, with coatedpapers the types and blending ratios of pigment and adhesive, weight ofcoating material, degree of smoothing and other factors are determinedin a manner achieving an optimal balance between gloss and printability.However, different technologies are needed to obtain high-gloss paperoffering excellent printability. In general, the gloss of coatedprinting paper improves in the order of fine-coated paper, coated paper,art paper, super art paper and cast-coated paper, with cast-coated paperoffering the highest gloss. In the present invention, the term “highgloss” means a level of gloss equivalent to or better than that of superart paper. Therefore, high-gloss paper means a coated printing paperhaving a gloss equivalent to or better than that of super art paper.

The conventional production methods of high-gloss paper include oneusing a cast coater. Under this method, a wet coating layer comprisingpigment and binder is pressure-bonded using a mirrored cast drum andthen heated and dried. This method has a drawback in that the productionspeed is reduced significantly compared with general art paper, coatedpaper and fine-coated paper.

A production method using a thermal calender, instead of a cast drum, isalso known. For example, Japanese Patent Application Laid-open No.56-68188 and Japanese Patent Publication Nos. 64-10638 and 64-11758describe methods to produce a coating layer by mixing pigment withpolymer latex or water-soluble polymer resin, applying and drying thecoating layer, and then treating the coating layer by heat calendering.In these published technologies, a polymer latex with a glass-transitiontemperature of 5° C., or 38° C. or above, is applied on a supportmaterial, and the obtained coating layer is treated with a thermalcalender at a temperature above the glass-transition temperature of thelatex used. This provides a simple and productive method that issuitable for producing normal coated paper. However, the method resultsin an insufficient gloss inferior to that of cast-coated paper and evenart paper. With thermal calendering, therefore, a level of glosscomparable to that of cast-coated paper cannot be achieved.

A yet another method is the one described in Japanese Patent ApplicationLaid-open No. 59-22683. Under this method, two or more copolymer latexesof different minimum film-formation temperatures are applied on a sheetor sheet having a pigment coating layer and then dried, after whichcalendering is applied as necessary to smoothen the surface. When dried,the latexes of different minimum film-formation temperatures willgenerate minute cracks on the surface of coated paper, thereby achievinggood ink absorbency without reducing gloss. The key point of thistechnology is to generate minute cracks on the surface of coated paper,and the drying condition must be given due attention in order to achievethis effect. In other words, the drying condition must be set so thatthe latex of the lower minimum film-formation temperature will meltcompletely while that of the higher minimum film-formation temperaturewill melt only partially. However, it is a common knowledge that dryingconditions are generally subject to fluctuations by a number of factors.When it comes to potential industrial applications of this technology,it is virtually impossible to maintain a uniform, constant dryingcondition throughout the production process. Therefore, under thistechnology it is extremely difficult to produce papers of stablequality.

As disclosed in Japanese Patent Application Laid-open Nos. 3-167396 and8-13390, the inventors found that a gloss equivalent to that of coatedpaper can be achieved without smoothing and that excellent printabilityin terms of ink absorbency and surface strength can also be achieved, bydesigning a coated printing paper that comprises a base material havinga support material and a pigment coating layer applied on top, whereinthe pigment coating layer has a surface layer made of a thermoplasticpolymer (emulsion comprising a polymer or copolymer exhibitingthermoplasticity) with a second order transition temperature of 80° C.;and the inventors also found that by calendering the aforementionedsurface layer at temperatures not exceeding the second order transitiontemperature of copolymer latex a high gloss equivalent to or better thanthat of super art paper can be achieved, together with sufficientlypractical levels of printability in terms of ink absorbency, surfacestrength and dot-error ratio, while eliminating deposits on the calenderrolls and thereby attaining higher productivity and manufacturingefficiency. However, these methods had problems in sheet-feed press,such as mottled ink impression, poor printability and insufficientseparation from the calender rolls.

SUMMARY OF THE INVENTION

In light of the situations described above, the purpose of the presentinvention is to provide a high-gloss coated printing paper offering highsheet gloss, excellent ink absorbency and surface strength, no mottledimpression of ink particularly in sheet-feed press, good ink-dryingproperty and excellent printability, as well as a method for producingsuch coated printing paper easily and at affordable cost by ensuring aconstant, stable quality.

The above issues were resolved by preparing a base material from asupport material and a pigment coating layer on top, wherein a surfacelayer containing a mixture of a thermoplastic polymer with aglass-transition temperature of 80° C. or above and a surface sizingagent is formed on the coating layer. This method would yield a coatedprinting paper offering high sheet gloss, excellent ink absorbency andsurface strength, no mottled impression of ink and excellentprintability. The surface layer proposed by the present invention shouldcomprise 100 weight-parts of thermoplastic polymer and 3 to 100weight-parts, or more preferably 3 to 50 weight-parts, or mostpreferably 3 to 20 weight-parts of surface sizing agent, in order toachieve a good balance of high gloss, mottled impression of ink andink-drying property.

The use of a thermoplastic polymer with a high glass-transitiontemperature and a surface sizing agent is a likely reason for the highgloss achieved by providing the surface layer proposed by the presentinvention, since they fill the concaves in the pigment coating layer andthereby optically smoothen the surface layer on the whole.

A surface layer consisting only of thermoplastic polymer causes ink tobe absorbed unevenly as it is transferred onto the paper. Therefore, inkdoes not dry uniformly and causes “trapping,” or a non-uniform transferof ink, in subsequent printing passes, which results in mottledimpression on the printed surface. On the other hand, a surface layerconsisting only of surface sizing agent offers poor printing efficiency,since ink does not dry quickly after being transferred onto the paper.The present invention eliminated mottled impression while achieving goodink-drying property, without sacrificing the characteristics affectingpaper quality, by combining a thermoplastic polymer with a highglass-transition temperature and a surface sizing agent.

The reason is explained as follows: A surface layer consisting only ofthermoplastic polymer has minute voids existing unevenly at the surfaceof the surface layer where the thermoplastic polymer particles retaintheir shape, and therefore such surface layer absorbs ink unevenly;whereas the same surface layer, when a surface sizing agent is added,will have the voids filled or covered by the surface sizing agent,thereby allowing ink to permeate more evenly and dry more quickly.

BEST MODE FOR CARRYING OUT THE INVENTION

The base material used in the present invention must comprise a basepaper and a pigment coating layer formed on top that contains pigmentand adhesive. The base paper can be a desired paper as long as a pigmentcoating layer can be formed on top. Examples include uncoated paperssuch as medium-grade paper, premium-grade paper, newspaper, single-sidegloss paper and special gravure paper. A pigment coating layer can beeasily formed on an uncoated paper using the normal production methodfor pigment-coated paper. Depending on the desired quality, however, thetypes and volumetric ratios of pigment and adhesive in the coatingmaterial should be changed as necessary.

Pigments that can be used in the pigment coating layer proposed by thepresent invention include conventional pigments including inorganicpigments such as kaolin, clay, delaminated clay, ground calciumcarbonate, precipitated calcium carbonate, talc, titanium dioxide,barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicate,colloidal silica and satin white, as well as organic pigments such asplastic pigment. These pigments can be used alone or in combination. Inthe present invention, it is preferable to blend 50 weight-parts or moreof kaolin for 100 weight-parts of pigment, in order to achieve greaterprintability while maintaining high gloss.

Adhesives that can be used in the pigment coating layer proposed by thepresent invention include adhesives conventionally used in theproduction of normal coated paper, including various copolymers such asstyrene-butadiene, styrene-acrylic, ethylene-vinyl acetate,butadiene-methyl methacrylate and vinyl acetate-butyl acrylatecopolymers; synthetic adhesives such as polyvinyl alcohol, maleicanhydride copolymer and acrylic methyl methacrylate copolymer; proteinssuch as casein, soybean protein and synthetic protein; starches such asoxidized starch, cationic starch, urea phosphate esterified starch,hydroxyethyl etherified starch and other etherified starches, anddextrin; and cellulose derivatives such as carboxyethyl cellulose,hydroxyethyl cellulose and hydroxymethyl cellulose. One or more of theseadhesives may be selected and used as necessary. Adhesive should accountfor 5 to 50 weight-parts, or preferably 5 to 25 weight-parts, withrespect to 100 weight-parts of pigment. Of particular preference is acombination of 13 weight-parts or less of styrene-butadiene copolymerlatex with 100 weight-parts of pigment, which will produce goodink-drying property. If necessary, appropriate amounts of dispersant,viscosity-increasing agent, water-retaining agent, defoaming agent,water-proofing agent, coloring agent, printability-improving agent andvarious other agents used in the coating material compositions for usein normal coated papers may be applied.

One or more pigment coating layers may be formed on one or both sides ofthe base paper. The weight of the coating layer proposed by the presentinvention should preferably be 2 to 40 g/m², or more preferably 5 to 25g/m², or most preferably 8 to 20 g/m², per side of the base paper.

The methods to apply a pigment coating layer on the base paper include adouble-roll size-press coater or gate-roll coater, blade-meteringsize-press coater or rod-metering size-press coater, sym-sizer or otherfilm-transfer roll coater, flooded-nip/blade coater, jet-fountain/bladecoater and short-dowel time-application coater. A rod-metering coaterusing grooved rods or plain rods instead of blades, curtain coater, diecoater or any other known coater may also be used.

In the present invention, after a pigment coating layer has been formedon the base paper, a mixed solution of thermoplastic polymer and surfacesizing agent is applied on top of the pigment coating layer as a surfacelayer. Before applying the mixed solution, the pigment coating layer maybe smoothened via a super calender, gloss calender, high-temperaturesoft-nip calender and so on.

Any thermoplastic polymer used in the above surface layer proposed bythe present invention should comprise emulsion particles of a polymer orcopolymer having thermoplasticity and a glass-transition temperature of80° C. or above, so that the particle shape will be retained afterhot-air drying or calendering. In the case of a core-shell polymer orcopolymer, the glass-transition temperature of the shell should be 80°C. or above. As long as a glass-transition temperature of 80° C. orabove is achieved, the types of monomers comprising the target polymeror copolymer, as well as the production method of the polymer orcopolymer, are of no concern. Examples of preferred component monomersinclude styrene and its derivatives, vinylidene chloride, and etheracrylate or methacrylate. There is no limitation as to how high theglass-transition temperature of thermoplastic polymer can be. Themaximum allowable glass-transition temperature of thermoplastic polymeris chiefly determined by the types of monomers and additives such asplasticizer used in the production of thermoplastic polymer, and isnormally around 130° C. When a polymer or copolymer with aglass-transition temperature of below 80° C. is used, the obtainedcoated paper will have poor gloss and leave deposits on the calenderrolls during calendering. Any thermoplastic polymer used in the presentinvention should ideally have an average particle size of 100 nm orless, in order to achieve high gloss and surface strength.

Surface sizing agents that can be used in the surface layer proposed bythe present invention include copolymer surface sizing agents such asstyrene-acrylic, styrene-maleic acid, styrene-methacrylate, olefin andurethane types. These sizing agents can be used alone or in combination.Sizing agents should be of solution or emulsion type and must not retainparticle shape after hot-air drying or calendering. A preferred averagemolecular weight of copolymer is 1000 to 500000. Among the copolymersurface sizing agents meeting these conditions, those ofstyrene-acrylic, olefin or styrene-maleic acid type are desirable. Inparticular, a styrene-acrylic sizing agent will achieve high sheetgloss.

In the present invention, a mixed solution of thermoplastic polymer andsurface sizing agent is applied on the pigment coating layer as asurface layer. As long as the purpose of the present invention is notcompromised in any way, a surface-layer coating solution may be preparedby adding, as necessary, a natural or synthetic resin adhesive forgeneral paper coating for the purpose of adjusting the strength of thecoated layer, a fluidity-adjusting agent or defoaming agent for thepurpose of adjusting the coatability of coating material, a lubricantfor the purpose of reducing deposits left on the calender or othercylinder rolls, and a coloring agent or small amounts of pigment for thepurpose of adding color to the surface of coating layer. Preferably thethermoplastic polymer and surface sizing agent should account for 80 to100 weight-percent of the overall surface layer in solid content. Thesurface-layer coating solution thus obtained will then be applied on thepigment coating layer as a surface layer. The coating weight can beadjusted as necessary to achieve desired properties. However, increasingthe coating weight excessively will not only increase cost, but it willalso reduce ink absorbency that leads to insufficient setting of ink andwill also reduce the strength of the surface layer. Given theseundesirable trends, it is not advisable to apply an excessive amount ofsurface layer. Normally a coating weight of approx. 0.1 g/m², orpreferably 0.3 to 3.0 g/m², per side of the base paper is sufficient.

The surface-layer coating solution may be applied using a blade coater,roll coater, air-knife coater, bar coater, gravure coater orflexo-coater, all of which are generally used in paper coating. Whenthermoplastic polymer and sizing agent are applied, as proposed in thepresent invention, no special conditions are necessary in the dryingprocess after coating. The normal drying condition used in theproduction of coated paper will provide an optimal surface layer. Thecoated printing paper thus obtained will offer, after calendering, ahigh gloss equivalent to or better than that of super art paper. Incalendering, a super calender, gloss calender or high-temperaturesoft-nip calender may be used alone or in combination. These calendermachines are normally used in the smoothing of coated paper. In thepresent invention, applying calendering at a metal roll temperature of100° C. or above, or even 150° C. or above, will still maintain goodseparation of the coated surface from the calender rolls. Even withoutcalendering, a coated printing paper offering a good sheet glossequivalent to that of glossy paper can be achieved.

EXAMPLES

The following is a detailed explanation of the present invention usingexamples. Note, however, that the invention will not be limited in anyway to the examples provided. Unless otherwise specified, the terms“parts” and “%” used in the examples indicate “weight-part(s) in solidcontent” and “weight-percent in solid content,” respectively.

(Production of Thermoplastic Copolymer A)

A four-neck flask equipped with an agitator, thermometer, cooling unit,dripping funnel and nitrogen-gas introduction tube was prepared, inwhich 300 parts of water, 9 parts of sodium dodecylbenzenesulfonate and4 parts of polyoxyethylene phenol ether (containing 10 mols of ethyleneoxide) were placed and mixed. Next, a monomer mixture comprising 80parts of styrene, 10 parts of α-methyl styrene, 100 parts of methylmethacrylate and 10 parts of methacrylate was prepared, and 60 parts ofthe mixture was placed in the flask. The temperature was raised to 60°C. concurrently with nitrogen replacement, and 7.2 parts of 20% aqueousammonium peroxide solution and 4.8 parts of 20% sodium bisulfurousanhydride solution were added and polymerized for 60 minutes. Then, 10parts of 20% aqueous ammonium peroxide solution was added and theremaining 140 parts of the monomer mixture was dripped over a period ofone hour. The resulting mixture was kept at 90° C. for four hours tocomplete the polymerization, and thus achieved an emulsion ofthermoplastic copolymer A. This emulsion had a solid content of 39%,glass-transition temperature of 107° C. and average particle size of 75nm.

(Thermoplastic Copolymer B)

A four-neck flask equipped with an agitator, thermometer, cooling unitand nitrogen-gas introduction tube was prepared, in which 310 parts ofwater, 5.6 parts of HITENOL N-08 (a polyoxyethylene nonylphenol ethersulfate manufactured by Dai-ichi Kogyo Seiyaku), 48 parts of styrene, 19parts of methyl methacrylate, 8 parts of ethyl methacrylate, 2.5 partsof divinyl benzene and 2.5 parts of methacrylate were placed. Thetemperature was raised to 70° C. concurrently with nitrogen replacement,after which 5 parts of 16% aqueous potassium peroxide solution was addedand kept at 85° C. for four hours to complete polymerization, and thusachieved an emulsion of thermoplastic copolymer B. This emulsion had asolid content of 21%, glass-transition temperature of 85° C. and averageparticle size of 75 nm.

(Thermoplastic Copolymer C)

Polystyrene emulsion with a glass-transition temperature of 100° C. andaverage particle size of 60 nm: Lytron 604 by OMNOVER

(Thermoplastic Copolymer D)

The same operation used to obtain thermoplastic copolymer A wasrepeated, except that 88 parts of styrene, 38 parts of methylmethacrylate, 70 parts of n-butyl methacrylate and 4 parts ofmethacrylate were used as the monomers, and obtained an emulsion ofthermoplastic copolymer D. This emulsion had a solid content of 39% andglass-transition temperature of 72° C.

(Surface Sizing Agent A)

Styrene-acrylic sizing agent (solution type): POLYMARON-NS-15-2 byArakawa Chemical Industries

(Surface Sizing Agent B)

Styrene-acrylic sizing agent (emulsion type): POLYMARON-NS-15-1 byArakawa Chemical Industries

(Surface Sizing Agent C)

Olefin sizing agent (solution type): POLYMARON 482S by Arakawa ChemicalIndustries

(Surface Sizing Agent D)

Styrene-maleic acid sizing agent: K-4 by Harima Chemicals

Production of Base Material (Pigment-Coated Paper)

A coating solution for coated paper with a solid content of 64% wasprepared from 70 parts of first-grade kaolin, 30 parts of particulateground calcium carbonate, 11 parts of styrene-butadiene copolymer latexand 5 parts of starch.

Using a blade coater operated at a coating speed of 500 m/min, theobtained coating solution was applied on both sides of a premium-gradecoating base paper with a grammage of 127 g/m², so that the dry weightof the coating layer on one side would become 14 g/m². After drying, abase material before surface coating (pigment-coated paper), having apigment coating layer with a moisture content of 5.5%, was obtained.

Example 1

A surface-layer coating solution with a solid content of 30% wasprepared from 100 parts of thermoplastic copolymer A, 15 parts ofstyrene-acrylic surface sizing agent A, 5 parts of lubricant ofpolyethylene-wax emulsion type and 10 parts of lubricant of fatty-acidderivative type.

Using a blade coater operated at a coating speed of 500 m/min, theobtained coating solution was applied on both sides of theaforementioned base paper (pigment-coated paper), so that the dry weightof the coating layer on one side would become 1.0 g/m². After drying, acoated paper with surface coating, with a moisture content of 6.5%, wasobtained. Then, the paper was treated on a super calender comprisingchilled rolls (65° C.) and cotton rolls, at a nip pressure of 180 kg/cmand speed of 10 m/min for 2 nips, to obtain a coated printing paper.

Example 2

A coated printing paper was obtained in the same manner as in Example 1,except that surface sizing agent A was changed to styrene-acrylicsurface sizing agent B.

Example 3

A coated printing paper was obtained in the same manner as in Example 1,except that thermoplastic copolymer A was changed to thermoplasticcopolymer B.

Example 4

A coated printing paper was obtained in the same manner as in Example 1,except that the paper was not given super calendering but treated on asoft calender comprising metal rolls (surface temperature: 180° C.) andelastic rolls, at a nip pressure of 130 kg/cm and speed of 270 m/min for2 nips.

Example 5

A coated printing paper was obtained in the same manner as in Example 1,except that surface sizing agent A was changed to olefin surface sizingagent C.

Example 6

A coated printing paper was obtained in the same manner as in Example 1,except that thermoplastic copolymer A was changed to thermoplasticcopolymer C.

Example 7

A coated printing paper was obtained in the same manner as in Example 1,except that surface sizing agent A was changed to styrene-maleic acidsurface sizing agent D.

Example 8

A coated printing paper was obtained in the same manner as in Example 1,except that the amount of surface sizing agent A was changed to 50parts.

Example 9

A coated printing paper was obtained in the same manner as in Example 1,except that the amount of surface sizing agent A was changed to 110parts.

Comparative Example 1

A coated printing paper was obtained in the same manner as in Example 1,except that only 100 parts of thermoplastic copolymer A was used.

Comparative Example 2

A coated printing paper was obtained in the same manner as in Example 1,except that only 100 parts of sizing agent A was used.

Comparative Example 3

A coated printing paper was obtained in the same manner as in Example 1,except that thermoplastic copolymer A was changed to thermoplasticcopolymer D.

Comparative Example 4

A coated printing paper was obtained in the same manner as in Example 1,except that 10 parts of surface sizing agent A was changed to 1 part ofviscosity-adjusting agent.

Comparative Example 5

A coated printing paper was obtained in the same manner as in Example 1,except that surface-layer coating solution was not applied on the basematerial (pigment-coated paper).

Example 10

A coated printing paper was obtained in the same manner as in Example 1,except that super calendering was not given.

Comparative Example 6

A coated printing paper was obtained in the same manner as in Example 1,except that surface-layer coating solution was not applied on the basematerial (pigment-coated paper) and that calendering was not given,either.

Table 1 shows the results of quality evaluation tests performed on thecoated papers obtained above.

TABLE 1 Thermo- Thermo- Thermo- Thermo- Vis- plastic plastic plasticplastic cosity- copolymer copolymer copolymer copolymer adjust- A Tg BTg C Tg D Tg Sizing Sizing Sizing Sizing ing 107° C. 85° C. 100° C. 72°C. agent A agent B agent C agent D agent Example 1 100 10 Example 2 10010 Example 3 100 10 Example 4 100 10 Example 5 100 10 Example 6 100 10Example 7 100 10 Example 8 100 50 Example 9 100 110 Comparative 100Example 1 Comparative 100 Example 2 Comparative 100 10 Example 3Comparative 100 1 Example 4 Comparative Base material (pigment coatinglayer) only Example 5 Example 10 100 40 Comparative Base material(pigment coating layer) only Example 6 Separa- tion Mottled Ink- Sheetduring Calen- im- drying gloss calen- dering pression property (%)dering Example 1 S/P ◯ ◯ 89 ◯ (65° C.) Example 2 S/P ◯ ◯ 89 ◯ (65° C.)Example 3 S/P ◯ ◯ 87 ◯ (65° C.) Example 4 HSNC ◯ ◯ 87 ◯ (180° C.) Example 5 S/P ◯ ◯ 82 ◯ (65° C.) Example 6 S/P ◯ ◯ 88 ◯ (65° C.) Example7 S/P ◯ ◯ 83 ◯ (65° C.) Example 8 S/P ◯ ◯ to Δ 89 ◯ (65° C.) Example 9S/P ◯ Δ 89 ◯ (65° C.) Comparative S/P X ◯ 89 Δ Example 1 (65° C.)Comparative S/P ◯ X 88 ◯ Example 2 (65° C.) Comparative S/P ◯ Δ 73 ΔExample 3 (65° C.) Comparative S/P X ◯ 76 Δ Example 4 (65° C.)Comparative S/P ◯ ◯ 73 ◯ Example 5 (65° C.) Example 10 Not ◯ ◯ 65 —given Comparative Not ◯ ◯ 45 — Example 6 given Reference: White-papergloss of OK Enamel White (Oji Paper), commercial cast-coated paper: 87%White-paper gloss of SA Kanefuji (Oji Paper), commercial super artpaper: 80%The evaluation items shown in Table 1 and respective test methods areexplained below:

-   -   Glass-transition temperature—Prepared a film from each sample at        20° C. and 65% (relative humidity) and calculated its        glass-transition temperature using the characteristic curve        obtained by measuring 20 mg of the film on a differential        scanning calorimeter (DSC6200R by Seiko Instruments) by raising        temperature at 5° C. per minute in a range from 0 to 100° C.    -   Sheet gloss—Measured on a Murakami gloss meter based on the 75°        reflection method.    -   Ink-drying property—Each sample was printed on using a RI-II        print tester, after which a white paper was pressed against the        printed surface to visually evaluate the transfer of ink onto        the white paper. Ink transfer was evaluated on three scales,        with “ο” indicating very small ink transfer onto the white        paper, “Δ” indicating small ink transfer and “X” indicating        significant ink transfer.    -   Mottled impression—Each sample was printed on at a speed of        8,000 sheets per hour on an offset four-color sheet-feed press        (R304 by MAN ROLAND), using HYECOO M (by Toyo Ink) in the order        of black, blue, red and yellow, and mottled impression of blue        color was evaluated visually.    -   Mottled impression was evaluated on three scales, with “ο”        indicating no mottled impression, “Δ” indicating slight mottled        impression and “X” indicating significant mottled impression.    -   Separation from calender rolls—Deposits left on the metal roll        surface was visually evaluated after calendering.

“ο” indicates good separation with no deposits left on the metal rollsurface, “Δ” indicates insufficient separation with slight deposits lefton the metal roll surface, and “X” indicates poor separation withsignificant deposits left on the metal roll surface.

As shown in Table 1, all coated printing papers obtained in accordancewith the present invention achieved high gloss. Printability in terms ofmottled impression and ink-drying property is also excellent or at apractical level.

On the other hand, those obtained in the comparative examples haveproblems, such as insufficient gloss, mottled impression and slow dryingof ink, and therefore do not meet the purpose of the present invention.

INDUSTRIAL FIELD OF APPLICATION

A coated printing paper provided by the present invention comprises abase material and a pigment coating layer formed on top, wherein asurface layer formed by applying and drying a mixed solution of athermoplastic copolymer with a glass-transition temperature of 80° C. orabove and a surface sizing agent is formed on the coating layer. Acoated printing paper provided by the present invention offers highgloss, no mottled impression, and sufficiently practical levels ofprintability in terms of ink-drying property and surface peel strength,and it also eliminates deposits on the calender rolls and thereby leadsto good productivity.

1. A coated paper for offset printing which is hot-air dried orcalendered and comprises a base paper and a coating layer comprisingpigment and adhesive formed on the base paper, wherein a surface layercomprising a thermoplastic polymer constituted by thermoplastic polymerparticles having a glass-transition temperature of 80° C. or above andan average particle size of 100 nm or less, and a surface sizing agentis formed on the coating layer, said coating layer and said surfacelayer being formed on the base paper in this order, the thermoplasticpolymer particles and surface sizing agent account for 80 to 100weight-percent of the overall surface layer in solid content, thesurface sizing agent is at least one copolymer selected from the groupconsisting of a copolymer of styrene-acrylic, a copolymer of olefins,and a copolymer of styrene-maleic acid, the thermoplastic polymerparticles remain as particles after hot-air drying or calendering,whereas the surface sizing agent is of a solution or emulsion type anddoes not remain as particles after hot-air drying or calendering,wherein in the surface layer, voids formed by the thermoplastic polymerparticles are filled or covered by the surface sizing agent, the weightof the surface layer is 0.1 to 3.0 g/m², and the surface layer contains3 to 100 weight-parts of the surface sizing agent for 100 weight-partsof the thermoplastic polymer particles.
 2. The coated paper for offsetprinting as described in claim 1, wherein the sizing agent is of asolution type.
 3. The coated paper for offset printing as described inclaim 1, wherein the copolymer constituting the surface sizing agent hasan average molecular weight of 1,000 to 500,000.